The brachial plexus is a group of 5 nerves from the spinal cord that provide the movement and sensation of an upper extremity. In some difficult deliveries, traction on the shoulder may lead to damage to the brachial plexus and will result in an arm that is paralyzed. This is called 'birth related brachial plexus injury' (BRBPI). This may occur in up to 1/1000 births and the nerves may be injured minimally to severely. About 2/3 of children with this injury will recover to quite functional levels simply by maintaining looseness of joints while their nerves slowly heal. Some children have nerve injuries severe enough that they require surgical reconstruction with nerve grafts and nerve transfers to achieve even adequate function. Even in children with otherwise "good" recovery, the motions of external rotation of the shoulder and supination of the forearm are weaker, later to recover, and often incomplete. More importantly, lack of full motion leads to long term changes in the structure, growth, and posture of the shoulder requiring further musculoskeletal surgery, or a child with permanent deformity or disability.